The Fires of a Red-Hot Reunion! - S16.E19

Ash and his friends come across the Kanto Fair, and seeing a Charmander perform on stage reminds him of his old friend Charizard. He tells Iris, Cilan and N about his and Charizard's history, with Misty and Brock making cameos during the tale. Professor Oak sends Charizard over, and after he meets Iris' Dragonite, the two trainers agree to have a friendly match. Ash's offer to let Charizard rejoin his team is met with approval from all parties involved.